1. Field of the Invention
The present invention relates to a device and a method for generating a secret key.
2. Description of the Related Art
A symmetric cryptographic system is a cryptographic system in which, in contrast to an asymmetric cryptographic system, all (legitimate) participants involved use the same key. The utilization of one and the same key for encrypting and decrypting data for computing and checking the message authentication codes, etc., involves that prior to any encrypted exchange the key itself must initially be distributed. However, since the safety of the entire method depends on the secrecy of the key, conventional approaches in most cases provide for the key exchange via a secure channel. This may take place, in particular, by manually introducing the key into the respective participants, e.g., by entering a password from which the actual key may then be deduced.
However, the key exchange via unsecure channels, which is known in cryptography as the “key distribution problem,” still presents a challenge to those skilled in the art. To solve this problem, the related art offers approaches such as the known Diffie-Hellman key exchange or the so-called hybrid encryption process which enable the exchange of symmetric keys by incorporating asymmetric protocols.
In the recent past, cryptographic systems are, however, increasingly discussed which deflect the problem of key establishment from the application layer of the OSI reference model to its bit transmission layer (physical layer, PHY). Approaches of this type are applied in the still young field of cyber-physical systems which distinguish themselves by predominantly using wireless and thus inherently unsafe communication channels.
Corresponding methods provide that each of the participating parties deduces a key from the physical characteristics of the channel which connects them in such a way that the keys generated in this way match without making it necessary for concrete parts of the key to be transmitted. U.S. Pat. No. 7,942,324 B1 provides a method of this type, as an example.
One essential prerequisite for the fact that the generated keys are of high quality (and thus secure) is that the generated keys include an approximately even distribution and maximum entropy. This, in turn, generally requires a sufficiently high variability of the transmission channel (or, in particular, of the channel parameters underlying the key generation). The variability of the relevant channel parameters may, however, differ a great deal and also vary timewise depending on the prevailing propagation conditions. In a wireless communication system, the time variance of the transmission channel is, for example, a function of the number of the scattered objects present between the transmitter and the receiver (and in their vicinity), among other things, as well as of the dynamic of the propagation scenario which may be completely static as well as very dynamic, e.g., when many objects—possibly including the transmitter or the receiver itself—are moving. Accordingly, it is, however, very difficult to optimally parametrize corresponding key generation processes a priori for the purpose of generating keys of sufficiently high quality within a minimum amount of time, since a compromise between the key generation rate and the key quality (in the sense of the key entropy) is required for this purpose.